Pen-down (shortened to pd) is used to trace the movements of turtles. The color of the pen is the same as the color of the turtle. While pen-down starts tracing, pen-up (shortened to pu) stops tracing a turtle’s movement. Think of pen-up and pen-down like a pen on the belly of a turtle; pen-down lowers the pen to the ground, leaving a mark of its path, while pen-up pulls the pen up, no longer marking the ground. Pen-down and pen-up are primitives used only with turtles. For example, ask turtles [ pen-down] would start drawing lines from the turtles’ movement, and ask turtles [ pen-up] would no longer trace the movement. (Pen-down, pen-up, and pen-erase are all equivalent to setting the turtle’s pen-mode to down, up, or erase)

The following code would make turtles draw squares and then move to a random location without leaving a trace:

ask turtles [
        pen-down
        repeat 4 [ forward 10 right 90 ]
        pen-up 
        setxy random-xcor random-ycor
]

Pen-erase (shortened to pe) erases any previously drawn lines when the turtle passes over them. Think of it as an eraser on the belly of the turtle.